MisHissippi Drainage System. — Westgate. 255 
the stream lines, probablj' because the thick sheet of drift every- 
where conceals the underlying rocks, except along the rivers. 
The rocks upon which they rest are often seen to be an eroded 
land surface. These facts show that the surface of Minnesota 
was nearly base-leveled when the Cretaceous beds were deposited. 
The surface of southern Minnesota is mostly level prairie formed 
by the drift. The Minnesota river cuts a trench in this prairie 
125 to 200 feet deep, and in doing so cuts in places through Cre- 
taceous beds and into the underlying rocks. The surface of the 
rocks upon which the Cretaceous beds lie must therefore be nearly 
level. It could not have been much more uneven at the time the 
Cretaceous beds were deposited upon it than it is at present, for 
any erosion which would have lowered the inequalities must have 
completely removed all the overlying softer Cretaceous deposits. 
Southern Minnesota therefore is considered to have been a nearly 
base-leveled countrj^ at the time that the Cretaceous deposits 
were laid down. The similarity between the present rock topog- 
raphy underlying the drift and the low relief of the land in 
Cretaceous time points to the conclusion that since Cretaceous 
time southern Minnesota has not been for any long period of time 
much elevated above base-level ; consequently post- Cretaceous 
erosion has not destroyed the Cretaceous base-level in the process 
of reducing the country a second time to base-level. 
5. The Cretaceous cycle in Arkansas. Evidence of the extea- 
sion of the Cretaceous base-level into Arkansas has been given 
b}' Grriswold.* The higher ridges of the novaculite area west of 
Little Rock reach an average elevation of 2,500 feet, perhaps 
marking the altitude of the upland from which the present topog- 
raphy has been cut. ' ' Still further south the ridge tops l>e- 
come broader and the valleys shallower. The appearance of an 
upland plain becomes more marked as the elevation decrease* 
southward and less opportunity is given for deep stream erosion. 
The southward sloping plain of Paleozoic rocks sinks below Cre- 
taceous or Tertiary strata and none of these older rocks are seei» 
again south of the contact line, projecting through the later sedi- 
ments. This gives evidence that the Paleozoic strata of this re- 
gion had been worn down to a prettj' level surface before the Cre- 
taceous strata were deposited. Thus we have altogether good 
* Geol. Survey of Arkansas. Ann. Report, 1890, vol. in. Novaculites 
by L. S. Griswold. pp. 220-222. 
